M42

M42 or M-42 may refer to:

In science:
* Messier 42 (M42), a nebula also called the Orion Nebula
* The 42nd known Mersenne prime

In transportation:
* M-42 (Michigan highway), a state highway in Michigan
* M42 motorway, a motorway in the United Kingdom
* BMW M42, a 1989 automobile piston engine
* Grand Central Terminal M42, a World War II era sub-basement and power station beneath New Yorks Grand Central Station train terminal

In photography:
* M42 lens mount, a standard for cameras

In firearms and military equipment:
* M42 Duster, a United States Army self-propelled anti-aircraft gun
* M42 (gas mask), a United States military gas mask
* M/42 (bicycle), a Swedish military bicycle
* 45 mm anti-tank gun M1942 (M-42), a Soviet anti-tank gun
* M42 Stahlhelm, a type of German WWII helmet
* M42, a Smith & Wesson hammerless revolver

In manufacturing:
* M42 grade of high speed steel with cobalt